Various types of chains including a roller chain generally employ steel for components such as inner link plates, outer link plates, and chain pins. With such steel chain, when components thereof reach durability limits due to repeated uses for an extended period of time, fatigue first occurs in the inner and outer link plates so that flexure deformation of the chain pin has a significant effect on fatigue fracture of the inner and outer plates.
FIG. 1 is a plan view of the main portion of a single-row roller chain for drive transmission. The roller chain includes a pair of parallel inner link plates 11, 11, a pair of parallel outer link plates 12, 12 which are disposed at a position outside of and displaced by one pitch from the inner link plates 11, 11, cylindrical bushings 13 which are disposed by fitting the ends thereof into the inner link plates 11, 11, cylindrical rollers 14 which are rotatably provided to the bushings 13 between the inner link plates 11, 11, and chain pins 15 which are inserted into the bushings 13 and disposed by fitting the ends thereof projecting from both ends of the bushings 13 into the outer link plates 12, 12.
FIG. 2 is a plan view of the main portion of the chain that shows conditions causing fatigue of the inner and outer link plates due to flexure deformation of the chain pins occurring in the single-row roller chain for drive transmission.
The roller chain undergoes a tensile loads in the directions of arrows a and b when being used. The tensile loads cause bending moments, shown in a stress distribution charts c and d, that are exerted to the chain pins 15 (hereinafter conveniently referred to as pin 15) so that the chain pins 15 warps in shapes illustrated by imaginary lines and the inner and outer link plates 11, 12 which are connected to the ends of the pins 15 deform in shapes indicated in imaginary lines. These result in breakdown of the inner and outer link plates 11, 12 due to accumulation of fatigue therein.
With such fatigue of the inner and outer link plates 11, 12, it has been known that if the pins 15 have a certain extent of residual compressive stress, the degree of flexure deformation of the pins 15 can be reduced so that the inner and outer link plates 11, 12 are able to withstand higher fatigue fracture limits. Therefore, standardized chains of ISO, JIS, ANSI, BS, and DIN use carburized pins which are made of carburized steel to have higher fatigue strength due to residual compressive stress. Thermal refined pins made of refined steel, i.e., medium carbon steel, are available as an alternative type of a chain pin (a chain pin is hereinafter conveniently referred to as carburized pin or refined pin as required). It has also been known that parts of the chain which have been heat-treated undergo surface treatment by shot peening.
However, the use of refined pins mentioned above is limited to a chain which is used a small number of repetitions at low speed and with a heavy load, since it has high creep rupture strength against impact loads, but low fatigue strength. Therefore, a trial is made to enhance the fatigue strength of the refined pin by changing the shape of the inner and outer link plates which are connected to each other by means of refined pins from a gourd shape having a narrow intermediate portion (Figure Eight Side Bar Style) into the shape of a cocoon having a wide waist (Wide Waist Figure Eight Side Bar Style) or an oval shape (Straight Side Bar Style). Steel balls with diameters slightly larger than those of the bushing holes of the heat-treated inner pin link plate and those of the pin holes of the heat-treated outer link plate are passed through the bushing holes and the pin holes, thereby giving residual compressive stress to the area around the holes of the inner and outer link plates so as to increase their fatigue strength. However, with a chain using the refined pins, not much satisfactory fatigue strength has been obtained as compared to a number of man-hours for production and the production costs.
On the other hand, the carburized pin with fatigue strength higher than the refined pin is suitable for the use as the chain which is operated repetitiously at high speeds, but has not so high enhancement of fatigue strength. Shot peening mentioned above serves as a soft shot for cleaning and finishing the surface of the parts through an approach such as removing the oxide coating film which are formed through heat treatment, and has not provided a maximum enhancement of fatigue strength. Thus, it is desirable that higher durability of the chain is achieved by enhancing residual compressive stress to increase the maximum allowable load.
The pins 15 of the roller chain shown in FIG. 1 is inserted into the bushings 13 which are installed between the inner link plates 11, 11. If the pin 15 and the bushing 13 are in a slide contact with each other for a long period of time, a clearance therebetween is widened due to sliding wear, thereby resulting in a stretched chain which is caused by accumulation of sliding wear between the chain pin and the bushing. Such stretch of the chain prevents smooth engagement of the chain with a sprocket, so that the stretch is believed to be limited to about 1.5 to 2.0%. Therefore, it is desired that a long life chain will be developed by making the stretch as short as possible.
The object of the present invention is to provide a chain having higher durability which is based on the enhancement in the maximum allowable load and reduction in friction resistance that are provided by enhancing residual compressive stress of the chain pin to reduce flexure deformation.